fix typo in mashup spec

[annieechen]

There's also a small red box from the html on line 477.

I think for the removeMarkers method, it also might be worth clarifying that you are or aren't supposed to delete the marker. I remember being confused as a student because the documentation says:

Note that the above method does not delete the marker. It simply removes the marker from the map. If instead you wish to delete the marker, you should remove it from the map, and then set the marker itself to null.